Founder and CEO of Facebook Mark Zuckerberg gives his speech during the presentation of the new Samsung Galaxy S7 and Samsung Galaxy S7 edge on February 21, 2016 in Barcelona, Spain. David Ramos/Getty Images

Facebook plans to let users delete messages they have sent from the inboxes of other users.

This feature carries the big risk of enabling abuse:Experts told Business Insider it could be used to hide evidence of harassment and to gaslight victims.

The feature was announced shortly after it was revealed that some of CEO Mark Zuckerberg's sent messages had been secretly deleted.

Facebook's recently announced plan to let users delete messages they have already sent to other people risks promoting harassment and abuse on the platform, experts have warned.

On Friday, the social network said that in the coming months it would introduce a feature to let users "unsend" messages sent via its Messenger app. The announcement came after TechCrunch revealed the day before that Facebook had been secretly deleting old messages sent by CEO Mark Zuckerberg from recipients' inboxes, sparking uproar over breaches of user trust and double standards for the privacy of Facebook executives versus its users.

But the experts Business Insider spoke to warned that the new feature risks enabling harassment and abusive behaviour on Facebook.

"Think about how many times you'veb gone back to chat records to make sure you actually told someone something. This might be in the midst of a totally trivial argument — 'I TOTALLY told you to get milk' or whatever," Florencia Herra-Vega, the CTO of encrypted messaging app Peerio, told Business Insider via message.

"But this kind of thing can become insidious and dangerous, which is where gaslighting comes in. If someone is harassing you, for example, or being abusive, and they can go back and modify or delete your conversations and then say they never behaved the way you accuse them of behaving, that's pretty awful!"

In an abusive relationship, someone might send their partner a message that isn't abusive in terms of content — but later delete it and deny having sent it, making their partner question themselves.

Cynthia Khoo, a Toronto-based lawyer working in internet policy and digital rights, wrote: "This is even more insidious when you consider that a lot of abuse and online harassment does not necessarily take the form of one explosive message, but a pattern of behaviour that results in a series of consistently harassing messages over a period of time, and it's the accumulation of them that causes harm, rather than necessarily any one of them on its own.

"So there's a potential there too of not just removing individual messages, but by removing messages selectively, making an entire pattern of interaction disappear and erasing the true nature of what it had been originally."

There are still some big unanswered questions

At this point, it's not clear how the feature will be implemented. Will both parties' messages be deleted when it is activated, or only one side? Will users be able to selectively delete messages after the fact or will expiry dates have to be set before sending? And will users be notified when it happens?

In the case of Zuckerberg, only his were deleted, messages were deleted selectively after the fact (not all recipients of messages from him say they have disappeared), and recipients do not appear to have been notified. (Facebook did not respond to Business Insider's request for comment.)

But whatever the implementation, it still risks causing issues because it would retroactively — and radically — change user expectations of how the platform should behave.

"If you're using Snapchat, you know your recipient will (probably) not save a copy of your message or video or whatever. But if you're using email, you have an understanding that both people will keep a copy of the correspondence," said Herra-Vega.

"The expectations around ownership and lifespan of a message are pretty integral to the functioning of a platform, and to people's trust and comfort with it, even if most people don't explicitly think about it that way."

'Honestly the thing I just keep thinking about is sexual harassers unsending evidence'

She added: "Honestly the thing I just keep thinking about is sexual harassers unsending evidence."

Khoo compared the announcement to past missteps by the company, and said she believed Facebook still hasn't properly learnt to consider "potential implications and consequences for marginalized communities."

"I would compare it to when they claimed to provide a tool to protect against non-consensual distribution of intimate images ("revenge porn"), and were widely panned because it involved victims having to upload the exact photos they're concerned about, to Facebook of all places," she wrote.

"Even now when so many of the concerns and warnings from academics and digital rights advocates have come through exactly as predicted, and facing the associated fallout, it's hard to believe that Facebook still hasn't learned that lesson."